The material of the photoconductive layer of an electrophotographic photosensitive member is required to have a high sensitivity, high S/N ratio, absorption spectral characteristic matching the spectral characteristic of electromagnetic wave to be irradiated, rapid optical responsibility, and high dark resistance, to be excellent in mechanical durability, and to be not harmful to the human body at the time of use.
The public attention has been focused on the use of hydrogenated amorphous silicon materials capable of satisfying the above requirements in electrophotographic photosensitive members. Electrophotographic photosensitive members having a photoconductive layer formed of such hydrogenated amorphous silicon material are disclosed, for example, in Japanese Unexamined Patent Publication No. 86341/1979. Various electrophotographic photosensitive members having an amorphous silicon photoconductive layer have been frequently used.
Japanese Unexamined Patent Publications Nos. 62254/1981 and 119356/1982 disclose the use of hydrogenated amorphous silicon materials containing carbon atoms in electrophotographic photosensitive members in order to improve their electrophotographic characteristics.
Incidentally, the formation of a film of such amorphous silicon material as above described as a constituent of the electrophotographic photosensitive member can be conducted by the sputtering process, film-forming manner by decomposing raw material gas with the action of thermal energy (that is, the so-called thermal-induced CVD process), film-forming manner by decomposing raw material gas with the action of light energy (that is, the so-called light-induced CVD process), or film-forming manner by decomposing raw material gas with the action of plasma (that is, the so-called plasma CVD process). Of these film-forming processes, the plasma CVD process has been frequently used. And there are known various apparatus suitable for practicing the plasma CVD process.
As the plasma CVD process, there is known the so-called microwave plasma CVD process based on microwave glow discharge decomposition. The microwave plasma CVD process has been practiced on an industrial scale.
The microwave plasma CVD process is more advantageous in comparison with other film-forming processes in the viewpoints that a relatively higher deposition rate and a relatively higher raw material gas utilization efficiency are attained. U.S. Pat. No. 4,504,518 discloses a microwave plasma CVD technique of making use of these advantages. The microwave plasma CVD technique described in this patent literature is directed to the formation of a high quality deposited film at a high deposition rate by practicing the microwave plasma CVD process at a reduced pressure of 0.1 Tort or less.
Japanese Unexamined Patent Publication No. 186849/1985 discloses a technique of improving the raw material gas utilization efficiency in the microwave plasma CVD process. The technique described in this publication is to improve the raw material gas utilization efficiency by arranging a substrate to circumscribe means for introducing microwave energy thereby forming an internal chamber (that is, a discharge space). Further, Japanese Unexamined Patent Publication No. 283116/1986 discloses a technique of improving the property of a deposited film formed by conducting the formation of the deposited film while controlling ion bombardment to the film deposited by applying a desired voltage through a plasma potential-controlling electrode (that is, a bias electrode) disposed in the discharge space.
U.S. Pat. No. 5,129,359 discloses a process for producing an electrophotographic photosensitive member based on these microwave plasma CVD techniques.
The process for producing an electrophotographic photosensitive member described in this U.S. Pat. No. 5,129,359 is practiced, for instance, by using a film-forming apparatus shown in FIG. 6(A) as a longitudinal section view and FIG. 6(B) as a cross section view.
In FIGS. 6(A) and 6(B), reference numeral 601 indicates a reaction chamber having a structure capable of being vacuum-sealed. Reference numeral 602 indicates a microwave introducing dielectric window made of a material (for example, quartz glass, alumina ceramics, or the like) which allows a microwave power to efficiently transmit into the reaction chamber 601 and can hermetically enclose the inside of the reaction chamber. Reference numeral 603 indicates a waveguide which serves to transmit a microwave power. The waveguide comprises a rectangular portion extending from a microwave power source (not shown) to the neighborhood of the reaction chamber and a cylindrical portion situated in the reaction chamber.
The waveguide 603 is connected to the microwave power source (not shown) through a stub tuner (not shown) and an isolator (not shown). Reference numeral 604 indicates an exhaust pipe. The exhaust pipe is open into the reaction chamber 601 through one end thereof and is connected to an exhaust device (not shown) through the remaining end thereof. Reference numeral 606 indicates a discharge space circumscribed by a plurality of substrates 605. Reference numeral 611 indicates a D.C. power source (a bias power source) which serves to apply a D.C. voltage to a bias electrode 612.
The process for producing an electrophotographic photosensitive member using the film-forming apparatus of the above-described constitution is conducted, for example, in the following manner. That is, the reaction chamber 601 is evacuated through the exhaust pipe 604 by operating a vacuum pump (not shown) to bring the inside of the reaction chamber 601 to a vacuum of 1.times.10.sup.-7 or less. The substrates 605 are then heated to and maintained at a temperature of 200.degree. to 300.degree. C. by means of heaters 607. Thereafter, raw material gases such as silane gas, hydrogen gas, and the like are introduced into the reaction chamber 601 through gas feed means (not shown). Then, a microwave power of 2.45 GHz from the microwave power source is introduced into the reaction chamber 601 through the waveguide 603 and the dielectric window 602. Simultaneously with this, the bias power source 611 electrically connected to the bias electrode 612 positioned in the discharge space 606 is switched on to apply a desired bias voltage between the bias electrode 612 and the substrates 605. In this case, the raw material gases in the discharge space 606 circumscribed by the substrates are excited and decomposed with the action of an energy of the microwave power, wherein ion bombardment is directed onto the substrates 605 by virtue of an electric field generated between the bias electrode 611 and the substrates 605, whereby a deposited film is formed on each of the substrates 605. During this film formation, each of the substrates 605 is rotated by revolving the rotary shaft 609 by means of a motor 610.
According to this process, it is possible to obtain electrophotographic photosensitive members having practically acceptable electrophotographic characteristics and which are satisfactory in terms of uniformity at a relatively low production cost. However, as for the electrophotographic photosensitive members produced by the conventional process, there are still remained problems which are required to be resolved. For instance, upon film formation by the conventional process, in the zone in which film formation is carried out at a relatively higher film-forming speed, it is difficult to stably obtain a deposited film which is homogeneous in terms of film quality, satisfies the requirements for optical and electric characteristics desired therefor, and is free of defects resulting in providing defective images upon image formation by the electrophotographic image forming process, at a high yield.
Specifically, as for the deposited film obtained in this case, it is often accompanied by a defect which leads to occurrence of uneven density for an image reproduced. The occurrence of uneven density for an image reproduced is not so problematic in the case of reproducing an original containing characters only. However, it is apparently problematic in the case of reproducing a halftone original such as a photograph, especially when the reproduction thereof is conducted at a high image-forming process speed. Further in the case of reproduction of a colored image for which demand has increased in recent years, it is required for an image reproduced to be precisely uniform in terms of the image density. In this case, the above occurrence of uneven density is a serious problem.
In addition, the electrophotographic photosensitive member comprising such deposited film does not satisfactorily comply with a demand in recent years for an improvement in the resolution for an image reproduced. The resolution of an image reproduced is governed by not only the electrophotographic photosensitive member but also the electrophotographic image-forming process including development and fixing steps which is employed upon the image formation. In recent years, a fine particle toner has been developed, and the electrophotographic image-forming process has been improved so as to make full advantage of such fine particle toner. Along with this, there is an increased demand for the electrophotographic photosensitive member to be improved so that an improvement is attained for the resolution for an image reproduced. However, making an electrophotographic photosensitive member comprising the above deposited film is difficult, to satisfy this demand.
European Patent Publication No. 454456 A1 proposes a technique of eliminating the above problems. Particularly, this patent publication discloses a light receiving member having a photoconductive layer composed of a non-single crystal silicon carbide containing fluorine atoms in a trace amount of 1 to 95 atomic ppm and oxygen atoms in a controlled amount in which the photoconductive layer is effectively relaxed in terms of internal distortion and is free of spherical growth defects at the surface and which is capable of preventing occurrence of "minute blank area", occurrence of "coarseness" and occurrence of "ghost" on an image reproduced. The technique described in this patent publication is aimed at diminishing the spherical growth defect at the surface of the light receiving layer of the light receiving member so as to stabilize and improve the quality of an image reproduced.
However, it is almost impossible to completely eliminate the appearance of such spherical growth defect at the surface of a photoconductive layer composed of a non-single crystal silicon carbide even by incorporating a prescribed amount of fluorine atoms and oxygen atoms thereinto. In fact, the present inventors prepared a light receiving member of the above constitution and subjected the light receiving member to continuous reproduction of a halftone original at an image-forming speed of 50 sheets per minute over a long period of time. As a result, there were found occurrence of uneven density and occurrence of a reduction in the resolution for the images reproduced after repetition of the copying shots. The causes for these problems are considered due to the spherical growth defect present at the surface of the photoconductive layer.
Japanese Unexamined Patent Publications Nos. 84965/1987 and 188665/1987 disclose a technique of eliminating the problems of an electrophotographic photosensitive member due to unevenness in the thickness thereof by grinding the surface of the electrophotographic photosensitive member. However, these patent publications do not describe anything about the interrelations between the surface grinding and the defects occurred on an image reproduced.
Incidentally, the present inventors found that the surface grinding technique described in these patent publications is not satisfactorily effective in making the electrophotographic photosensitive member such that it stably provides a high quality reproduced image excelling in resolution and density uniformity upon high speed image reproduction.
In order to stably obtain a high quality reproduced image excelling in resolution and density in high speed image reproduction, due care should be made so that no problem is occurred due to reflection of light used for the exposure. For instance, in the case of conducting image reproduction using an amorphous silicon series photosensitive member as the photoreceptor in the digital copying machine in which a semiconductor laser is used as the exposure light source, there is used as the semiconductor laser a near infrared laser having an energy which is lower than the energy band gap of the amorphous silicon film of the photosensitive member, wherein the laser rays are not completely absorbed by the amorphous silicon film and the residual laser rays other than those absorbed by the amorphous silicon film are transmitted or reflected. In this case, the laser rays reflected at the surface of the photosensitive member often interfere with the laser rays reflected at the interface between the amorphous silicon film and the substrate or the layer interface of the amorphous silicon film to provide an interference fringe pattern on an image reproduced.
U.S. Pat. No. 4,808,504 discloses a technique of eliminating the problem of providing such interference fringe pattern on an image reproduced. Particularly, this patent literature describes an electrophotographic photosensitive member comprising a substrate having an uneven-shaped surface composed of a plurality of spherical dimples and a light receiving layer disposed on said uneven-shaped surface of the substrate in which interference fringes occurred are dispersed within the spherical dimples to prevent images reproduced from being accompanied by interference fringe patterns.
This electrophotographic photosensitive member has been evaluated as being effective to prevent the occurrence of an interference fringe pattern on an image reproduced. However, the electrophotographic photosensitive member is disadvantageous in the viewpoint that the production cost thereof unavoidably becomes remarkable because specific facility and process are required for forming the uneven-shaped surface composed of a plurality of spherical dimples at the surface of a substrate in the preparation of the electrophotographic photosensitive member.